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Large flowers often contain larger nectar rewards, and receive more pollinator visits, than small flowers. We studied behavioural mechanisms for the formation of flower preferencce in bumblebees in a two-phase laboratory experiment. Flower-naive Bombus terrestris (L.) foraged on artificial flowers that bore either a big (3.8 cm diameter) or a small (2.7 cm diameter) display of a uniform colour. Only flowers of one display size contained nectar rewards. We changed the display colour and the locations of big and small flowers in the second experimental phase. We recorded the bees' choices in both trials. 41% of the bees made their first visit to a small flower. The bees learned to associate display size with food reward, and chose rewarding flowers with > 85% accuracy by the end of each learning trial. Some learning occured within the bees' first three flower visits. Learning of the size-reward association was equally good for big and small displays in the first trial, but better for small displays in the second trial. Formation of size-reward associations followed a similar course in both trials. This suggests that the bees did not apply their experience from the first learning trial to the new situation of the second trial. Rather, they treated each phase of the experiment as an independent learning trial. We suggest that pollinators from flower-size preferences through associative learning, and that they may not transfer the concept of "flower size" from one situation to another. Implications for the possible evolution of floral displays are discussed.